This invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves of the CHA framework type. In particular, the present invention relates to the synthesis of aluminophosphate and silicoaluminophosphate molecular sieves of the CHA framework type using N-methylethanolamine as synthesis template.
Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstocks. It has been known for some time that oxygenates, especially alcohols, e.g. methanol, are convertible into light olefin(s). The preferred methanol conversion process is generally referred to as the methanol-to-olefin(s) (MTO) process, where methanol is converted to primarily ethylene and propylene in the presence of a molecular sieve.
Some of the most useful molecular sieves for converting methanol to olefin(s) are the metalloaluminophosphates such as the silicoaluminophosphates (SAPO""s). There are a wide variety of SAPO molecular sieves known in the art. Of these, the more important examples include SAPO-5, SAPO-11, SAPO-18, SAPO-34, SAPO-35, SAPO-41, and SAPO-56. For the MTO process, SAPO molecular sieves having the CHA framework type (xe2x80x9cAtlas of Zeolite Framework Typesxe2x80x9d, 2001, 5th Edition, p.96) and especially SAPO-34 are particularly important catalysts. The CHA framework type has a double six-ring structure in an ABC stacking arrangement. The pore openings of the structure are defined by eight-member rings that have a diameter of about 4.0 xc3x85 and cylindrical cages within the structure of approximately 10xc3x976.7 xc3x85. Other SAPO molecular sieves of CHA framework type include SAPO-44, SAPO-47 and ZYT-6.
The synthesis of AlPO4 and SAPO molecular sieves is a complicated process. A number of variables need to be controlled to optimise the synthesis in terms of the purity, yield, and quality of the molecular sieve produced. Of these variables, the choice of synthesis template is usually one of the most important in determining which framework type is obtained.
One desirable group of SAPO molecular sieves are those which have low silicon content. SAPOs of the CHA framework type with low silicon content are particularly desirable for use in the MTO process. Wilson, et al., reported that it is beneficial to have lower Si content for methanol-to-olefins reaction (Microporous and Mesoporous Materials, 29, 117-126, 1999). Low Si content has the effect of reducing propane formation and decreasing catalyst deactivation.
AlPO4 and SAPO molecular sieves having the CHA framework type may be represented as [Alxe2x80x94Pxe2x80x94O]xe2x80x94CHA and [Sixe2x80x94Alxe2x80x94Pxe2x80x94O]xe2x80x94CHA according to nomenclature proposed by L. B. McCusker et.al. (Proposal to IUPAC; L. B. McCusker, F. Liebau and G. Engelhardt, Pure Appl. Chem. 73 (2) 381-394, 2001).
In U.S. Pat. No. 4,440,871, (Lok et.al) the synthesis of a wide variety of SAPO materials of various framework types is described with a number of specific examples. Also disclosed are a large number of possible organic templates, with some specific examples. In the specific examples, a number of CHA framework type materials are described. SAPO-34 is prepared utilizing tetraethylammonium hydroxide (TEAOH) or isopropylamine, or mixtures of TEAOH and dipropylamine (DPA). This is believed to be the first reported synthesis of a SAPO having the CHA framework type. Also disclosed in this patent is a specific example that utilizes cyclohexylamine in the preparation of SAPO-44. Although other template materials are described in this patent, there are no other templates indicated as being suitable for preparing SAPO""s of CHA framework type. Certain aminoalcohols are mentioned, including triethanolamine, N-methyldiethanolamine, N-methylethanolamine, N,N-dimethylethanolamine and N,N-diethylethanolamine as possible templates for SAPO molecular sieves. Of these materials, N,N-diethylethanolamine is shown to produce SAPO-5, which is of framework type AFI. For the other aminoalcohols, no indication is provided as to which SAPO or which fiamework type may be obtained through their use.
Since the synthesis of SAPO-34 was reported in U.S. Pat. No. 4,440,871, TEAOH, either alone or in combination with DPA, has been the template of choice for preparing SAPO having the CHA framework type. However, there are problems associated with the use of TEAOH and DPA. When used alone, TEAOH affords a limited range of synthesis parameters. For example, under certain conditions, TEAOH will also template the synthesis of SAPO-18, which has the AEI framework type. TEAOH is thus relatively intolerant to synthesis condition variations. TEAOH is sometimes combined with DPA. However, DPA has a low boiling point (110xc2x0 C.), resulting in the need for production plants that can handle high pressures. In certain countries, the use of DPA requires special regulatory authorizations due to its toxicity. Also, DPA is an aggressive template and is often implicated in re-dissolution of the SAPO molecular sieve during its synthesis, resulting in poor quality crystalline product due to surface pitting of the crystals.
In U.S. Pat. No. 4,440,871, it was reported that SAPO-44 was obtained xe2x80x9cas the major phasexe2x80x9d using cyclohexylamine as template. In U.S. Pat. No. 6,162,415 (Liu, et.al.), relatively pure CHA SAPO-44 was obtained using the same template but with control of the ratio of template to aluminum component and the ratio of phosphorous component to aluminum component.
In European Patent Publication No. 0,993,867, it was reported that the use of methylbutylamine resulted in SAPO-47, and the use of cyclohexylamine resulted in impure SAPO-44. Methylbutylamine has an even lower boiling point, at 91xc2x0 C., than DPA.
In U.S. Pat. No. 4,861,739 (Pellet, et al.), Example 102, it was reported that the use of N,N-diethylethanolamine produced CoAPSO-47, having Si concentrated on the peripheries of the crystal and Co at the center.
In U.S. Pat. No. 4,310,440 (Wilson et.al), triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, N-methyldiethanolamine, and N-methylethanolamine, were all used to prepare AlPO-5, aluminophosphates of framework type AFI. N-methylethanolamine was also reported to produce AlPO4-21 of framework type AWO.
In European Patent Publication No. 0,993,867, it was reported that diethanolamine produced SAPO-34 and SAPO-5 under different synthesis conditions.
In the art, various attempts have been made to improve the synthesis of AlPO4 or SAPO molecular sieves. One approach has been the use of fluoride addition to the synthesis.
In U.S. Pat. No. 5,096,684 (Guth et.al.), morpholine and TEAOH were found to template the production of SAPO-34 when in the presence of HF. According to Guth et.al., the use of HF in combination with the organic template results in SAPOs which have improved thermal and hydrolytic stability.
In U.S. Pat. No. 4,786,487 (Kuehl et.al.), SAPO-20 was produced from synthesis mixtures containing tetramethylammonium hydroxide and fluoride ions from water soluble sources of fluoride such as Na, K and ammonium fluoride.
In U.S. Pat. No. 6,001,328 (Lillerud et.al.), SAPO indicated as UiO-S7 was prepared using tetramethylammonium hydroxide pentahydrate or tetramethylammonium hydroxide, in combination with HF.
In a Ph.D. thesis (E. H. Halvorsen, University of Oslo, 1996), it was reported that low silica SAPO-34, designated as UiO-S4, was produced using TEAOH template in combination with HF.
As can bee seen from the disclosures described herein, there have been a number of attempts to find altemative synthesis templates for the CHA framework type with limited success. It is desirablc therefore to find new synthesis templates, template systems and synthesis conditions that are specific for the synthesis of CHA framework type. In addition, there is a need for new templating systems which afford more effective control of the final composition of the SAPO molecular sieve materials and in particular the Si content of the final product. In addition, a further need is to obtain SAPO materials having the CHA framework type that have a low acid density and a low silica content. There is also a need for synthetic methods that produce aluminophosphates and/or silicoaluminophosphates in relatively high yields.
The present invention provides a process for preparing microporous crystalline aluminophosphate (xe2x80x9cAlPOxe2x80x9d) or SAPO molecular sieves of CHA framework type, the process comprising:
(a) forming a reaction mixture comprising a source of aluminum, a source of phosphorous, optionally a source of silicon and at least one template, the template comprising N-methylethanolamine;
(b) inducing crystallization of AlPO and/or SAPO from the reaction mixture, and
(c) recovering aluminophosphate and/or silicoaluminophosphate molecular sieve,
wherein the mole ratio of template to aluminum in the reaction mixture is greater than 0.5.
The process of the present invention results in new molecular sieve materials, which, as synthesized, have unique X-ray diffraction (XRD) patterns. Thus, the process of the present invention provides for a variety of new materials that have the XRD patterns as described below.
In one embodiment, there is provided an AlPO or SAPO molecular sieve, substantially of CHA framework type, comprising within its intra-crystalline structure N-methylethanolamine.
In another embodiment, there is provided a composition of a microporous crystalline SAPO molecular sieve, which has a characteristic XRD pattern containing at least the d-spacings as set forth in Table I:
It is preferred that the composition as identified in Table I comprises N-methylethanolamine within its intra-crystalline structure.
In a further embodiment, there is provided an AlPO or SAPO molecular sieve, substantially of CHA framework type, comprising within its intra-crystalline structure N-methylethanolamine and fluoride.
In a further embodiment, there is provided a composition of a microporous crystalline SAPO molecular sieve which has a characteristic XRD pattern containing at least the d-spacings as set forth in Table II:
It is preferred that the composition as identified in Table II comprises N-methylethanolamine and fluoride within its intra-crystalline structure.
In a further embodiment, the present invention provides a process for preparing microporous crystalline AlPO or SAPO molecular sieves of CHA framework type, in which process a source of fluoride is present in the reaction mixture in combination with N-methylethanolamine.
In another embodiment, the present invention provides a method for the manufacture of a formulated catalyst composition, the method comprising forming a mixture comprising at least one microporous crystalline SAPO molecular sieve of CHA framework type comprising within its intra-crystalline structure N-methylethanolamine or as obtained from a process utilizing a template comprising N-methylethanolamine, with at least one formulating agent, to form a catalyst composition. In a further embodiment, fluoride is also present in the intra-crystalline structure.
In yet a further embodiment, the present invention provides for a formulated molecular sieve composition comprising at least one microporous crystalline SAPO molecular sieve of CHA framework type comprising within its intra-crystalline structure N-methylethanolamine or as obtained from a process utilizing a template comprising N-methylethanolamine, in admixture with at least one formulating agent. In a further embodiment, fluoride is also present in the intra-crystalline structure.
In a further embodiment, the present invention provides for the use of a template comprising N-methylethanolamine in the synthesis of AlPO or SAPO of CHA framework type. In a further embodiment, the present invention provides for the use of a template comprising N-methylethanolamine in combination with a source of fluoride ions in the synthesis of AlPO or SAPO of CHA framework type.